Elevated platelet counts are a common finding in many cancer patients, including patients with ovarian cancer. Patients with ovarian cancer and thrombocytosis have a worse prognosis compared to patients with similar stages of cancer and normal platelet counts. We have shown that platelets promote proliferation of cancer cells both in vitro and in the murine models of ovarian cancer; and reducing platelet counts decreased the size of orthotopic tumor induced in mice by ovarian cancer cells. To identify the mechanisms of the growth- enhancing effect of platelets on cancer cells, we used blocking reagents against platelets in vitro, and found that platelet activation and release of TGF1 are important for the proliferative effect of platelets on cancer cells. In this project, we will study the interaction between platelets and cancer cells in vivo, using both murine models of ovarian cancer and tissue samples obtained from patients with ovarian cancer. Our hypothesis is that there is a feedback loop between platelets and cancer cells. Cancer cells secrete ADP and activate platelets, and platelets secrete TGF1 that promotes proliferation in cancer cells. In the specific aim 1, we will investigate whether blocking ADP receptors on platelets would disrupt the growth promoting effect of platelets on orthotopic tumors in mice, using genetically modified mice or pharmacologic reagents. In the specific aim 2, we will target TGF1 secretion from platelets, TGF1 receptor on cancer cells, or TGF1 receptor signaling to evaluate the role of TGF1 on the platelet-cancer cell interaction. We will use platelet-specific TGF1 deficient mice, inhibitor RNAs, or pharmacologic reagents against TGF receptor signaling to conduct these experiments. For the interaction between platelets and cancer cells to occur inside the tumors, platelets should exit circulation and enter into tumor microenvironment. We have shown the presence of platelets outside of blood vessel inside the implanted tumors in mice. In the specific aim 3, we will study the mechanisms of platelet exit from tumor microcirculation using immunofluorescence and electron microscopy on human and murine ovarian cancer tissue samples. We will identify the route of platelet extravasation, and evaluate the dependency of platelets on neutrophils for extravasation. The goal of our studies in this grant proposal is to evaluate the possibility of using anti-platelet reagents as an effective anticancer therapy.